Summary. 1. (1) A small area where sleeping sickness was endemic in Equatoria Province of the Anglo-Egyptian Sudan was selected for experimental trials of the Symes Block method of tsetse fly control. The area is described. 2. (2) The disease was first recorded in this district in 1918. Since that date various methods of controlling it have been tried and these are described. 3. (3) The application of the Block method of tsetse fly control is detailed: together with the results of the experiment on the fly and the disease incidence. 4. (4) The effectiveness of the experiment in tsetse fly control is discussed with particular reference to its advantages over the methods previously used in this area.